CMOS current mode RF detector and method

ABSTRACT

An integrated detector circuit ( 20 ) includes first and second gain stages (GS 1 , GS 2 ). The first gain stage has an input ( 82 ) that monitors a high frequency signal (V RFDET ) for routing a first detection current (IS 1 ) to a node ( 60 ). The second gain stage includes a first current source (PF 1 ) that supplies a bias current (I MAX1 ) indicative of a predefined amplitude of the high frequency signal. An input of the second gain stage monitors the high frequency signal to route a portion of the bias current to the node as a second detection current (IS 2 ), which is limited to the bias current when the high frequency signal is greater than the predefined amplitude to compensate for a nonlinearity in a transconductance of the second gain stage.

BACKGROUND OF THE INVENTION

[0001] The present invention relates in general to semiconductor devices and, more particularly, to high frequency detector circuits.

[0002] In most if not all countries, the transmission of high frequency signals is controlled by industry standards and/or by government regulation. For example, transmitted signals in cellular telephones and other wireless communication devices are limited to operating in predefined frequency bands ranging from about four hundred megahertz to about 2.5 gigahertz. The amplitudes of the transmitted signals also are limited to avoid interference with other devices operating in the same or adjacent frequency bands. In addition, manufacturers extend the battery life of the wireless devices by reducing the transmitted power when such a reduction is possible without interrupting communications.

[0003] In order to control the transmitted power, a high frequency signal typically is monitored with a radio frequency (RF) detector that senses the signal's amplitude and produces a corresponding detection signal. A control circuit uses the detection signal and other communications information to control a power amplifier that produces the transmitted signal at an appropriate amplitude.

[0004] Previous systems use discrete devices such as Schottky diodes for the RF detectors. However, the discrete devices require additional circuitry to generate a useful detection signal, which increases the system's parts count as well as its size and manufacturing cost. Other systems integrate the Schottky diodes with the control circuitry to form a single integrated circuit. However, this scheme increases the complexity of the fabrication process, which increases the fabrication cost of the integrated circuit and can reduce the die yield, further increasing the cost.

[0005] Hence, there is a need for an integrated RF detector circuit and detection method that reduces the physical size and manufacturing cost of a wireless communications system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a block diagram of a power stage of a wireless communications device; and

[0007]FIG. 2 is a schematic diagram of a radio frequency signal detection circuit.

DETAILED DESCRIPTION OF THE DRAWINGS

[0008] In the figures, elements having the same reference number have similar functionality.

[0009]FIG. 1 is a block diagram of a power stage 10 of a wireless communications device such as a cellular telephone, including a microprocessor 11, a control circuit 13, a power amplifier 18 and a coupler 25. Power stage 10 generates a high frequency signal V_(RF) with a carrier frequency component operating at a radio frequency (RF) of about nine hundred megahertz. Other embodiments operate with a high frequency signal V_(RF) that ranges between about four hundred megahertz and about 2.5 gigahertz.

[0010] Microprocessor 11 is a digital processing circuit that includes a digital to analog converter for converting internal digital data to an analog amplitude control signal V_(MAG) to set the magnitude of high frequency signal V_(RF).

[0011] Power amplifier 18 has a signal input 23 that receives an RF signal V_(RFIN) at a carrier frequency of about nine hundred megahertz that is modulated with analog and/or digital information. Power amplifier 18 amplifies V_(RFIN) to produce high frequency signal V_(RF) at a node 24 with a power level that can range over about five decades, or an equivalent voltage range from about twenty millivolts to about four volts. An analog amplitude control signal V_(CTRL) is produced at a node 17 by control circuit 13 to control the gain of power amplifier 18 and therefore the amplitude of high frequency signal V_(RF).

[0012] Coupler 25 is an attenuator that transfers high frequency signal V_(RF) to a transmission line on a node 19 to produce a high frequency signal V_(RFDET) at a reduced amplitude. In one embodiment, coupler 25 attenuates V_(RF) by a factor of ten in order to avoid driving control circuit 13 into an overload condition when V_(RF) has a high amplitude level.

[0013] Control circuit 13 includes a transconductance amplifier 14, a differential transimpedance amplifier 16 and an RF detector 20. Control circuit 13 is formed on a semiconductor die as an integrated circuit fabricated with a standard complementary metal-oxide-semiconductor (CMOS) process using 0.6 micrometer lithography.

[0014] Transconductance amplifier 14 amplifies and converts amplitude control signal V_(MAG) to a proportional current I_(REF) that is provided on a node 15 to function as a reference current.

[0015] RF detector 20 has a sense input on node 19 for detecting or sensing the amplitude of high frequency signal V_(RFDET) to produce a direct current (DC) detection current I_(DET) at an output coupled to a node 21.

[0016] Differential transimpedance amplifier 16 amplifies I_(DET) using current I_(REF) as a reference and produces V_(CTRL) at node 17.

[0017] RF detector 20 is formed with nonlinear elements which are needed to produce DC components of detection current I_(DET) to provide the detection function. RF detector 20 compensates for the nonlinear elements to produce detection current I_(DET) as a substantially linear function of the amplitude of V_(RFDET) as described in detail below.

[0018]FIG. 2 is a schematic diagram showing RF detector 20 in further detail, including a series of parallel-connected transconductance gain stages GS₁, GS₂ . . . GS_(N), where N is an integer. In one embodiment, N=5. To prevent standing waves on node 19, a termination resistor 71 is used to match the characteristic impedance of the transmission line designated as node 19. In one embodiment, resistor 71 has a value of about fifty ohms. V_(RFDET) is coupled through a coupling capacitor 80 to a node 82. In one embodiment, coupling capacitor 80 has a value of about ten picofarads. A DC bias voltage V_(BIAS) is coupled through a resistor 72 to provide a DC bias on node 82. Resistor 72 typically has a value at least ten times that of resistor 71 to avoid loading node 19 and attenuating high frequency signal V_(RFDET).

[0019] Gain stage GS₁ includes a p-channel metal-oxide-semiconductor field effect transistor (MOSFET) PF₁ and an n-channel MOSFET NF₁ configured as shown. The other gain stages are similarly configured so, for example, gain stage GS₂ includes a p-channel MOSFET PF₂ and an n-channel MOSFET NF₂, gain stage GS_(N) includes a p-channel MOSFET PF_(N) and an n-channel MOSFET NF_(N), and so forth.

[0020] A diode coupled p-channel transistor 79 receives a reference current I_(REF2) from a current source 78 to provide a gate bias potential that establishes maximum currents I_(MAX1) through I_(MAXN) that can flow through MOSFETs PF₁-PF_(N), respectively. Transistor 79 is scaled or matched with MOSFETS PF₁-PF_(N), so maximum currents I_(MAX1) through I_(MAXN) are multiples of I_(REF2).

[0021] MOSFETs NF₁-NF_(N) respectively have first conduction electrodes or sources coupled to a common node 60 and control electrodes or gates coupled to node 82 to receive bias voltage V_(BIAS). The effective sizes or widths W₁-W_(N) of MOSFETs NF₁-NF_(N) are scaled in predefined ratios and since NF₁-NF_(N) have the same gate to source potential, quiescent currents I0₁-I0_(N) flow through MOSFETs NF₁-NF_(N) to common node 60 in the same ratios.

[0022] Common node 60 is maintained at a substantially constant potential by a regulation circuit that includes transistors 73 and 76, a current source 74 and an amplifier 75. Transistor 73 is scaled to MOSFETs NF₁-NF_(N) and also has its gate receiving bias voltage V_(BIAS). Current source 74 supplies a reference current I_(REF1) that is scaled to reference current I_(REF2) to establish a reference potential V_(REF1) at an inverting input of amplifier 75. A feedback path from the gate of transistor 76 to common node 60 and the non-inverting input of amplifier 75 maintains the potential of common node 60 substantially constant. As a result, quiescent currents I0₁-I0_(N) have controllable values which are scaled to reference current I_(REF1).

[0023] In addition to quiescent currents I0₁-I0_(N), MOSFETs NF₁-NF_(N) produce detection currents IS₁-IS_(N), respectively, in response to high frequency signal V_(RFDET). Quiescent currents I0₁-I0_(N) and detection currents IS₁-IS_(N) are summed on node 60, routed through a transistor 76 and mirrored in a transistor 77 to produce detection current I_(DET) at node 21 as an output current of RF detector 20.

[0024] Recall that MOSFETs PF₁-PF_(N) are scaled or ratioed to provide limiting or maximum currents that can flow in gain stages GS₁-GS_(N). MOSFETS NF₁-NF_(N) are also scaled or ratioed to produce quiescent currents I0₁-I0_(N) that establish the operating points of gain stages GS₁-GS_(N). The effect of this scaling is to define amplitude breakpoints of high frequency signal V_(RFDET) at which detection currents IS₁-IS_(N) reach their respective maximum current levels I_(MAX1)-I_(MAXN). Table I below shows the effective width W_(X) of MOSFET NF_(X), quiescent currents I0_(X) through gain stage GS_(X), the effective width WP_(X) of MOSFET PF_(X), maximum current I_(MAXX) and the voltage amplitude of high frequency signal V_(RFDET) at which (I0_(X)+IS_(X))=I_(MAXX) for values of X from 1−N in a standard 0.6 micrometer CMOS embodiment in which N=5. TABLE I V_(RFDET) when Gain I_(MAXX) = Stage W_(X) (um) I_(MAXX) (A) IO_(X) IO_(X) + IS_(X) WP_(X) GS₁ 350 um   35 uA   30 uA  40 mV   37 um GS₂  78 um 11.5 uA  6.7 uA  110 mV 11.5 um GS₃  45 um   19 uA 3.85 uA  320 mV   19 um GS₄  17 um   43 uA 1.45 uA  890 mV   43 um GS₅  7 um  120 uA  0.6 uA 2500 mV  120 u

[0025] The operation of gain stage GS₁ proceeds as follows. MOSFET NF₁ routes a portion of bias current I_(MAX1) to common node 60 as detection current IS₁ in response to high frequency signal V_(RFDET). When V_(RFDET) reaches an amplitude of about forty millivolts, IS₁=(I_(MAX1)−I0₁) and does not increase with further increases in V_(RFDET). Gain stages GS₂-GS₅ have similar operation, but their detection currents reach maximum values at different breakpoints as shown in Table I.

[0026] The transfer functions of gain stages GS₁-GS_(N) are nonlinear over the full amplitude range of V_(RFDET) from zero to about four volts, which is necessary for producing detection currents IS₁-IS_(N) with DC current components. For example, gain stage GS₁ produced IS₁ as a function of V_(RFDET) in accordance with a transconductance gm₁ given by gm₁=(IS₁/V_(RFDET))=k*W₁*V_(RFDET), approximately, where K is a constant and W₁ is the effective width of MOSFET NF₁. Note that transconductance gm₁ is proportional to W₁, which is a constant, and to V_(RFDET), which is a variable. Since gm₁ increases with the amplitude of V_(RFDET), IS₁ is a nonlinear function of V_(RFDET). Gain stages GS₂-GS_(N) have similar transfer functions.

[0027] The detailed operation of RF detector 20 is described as follows when N=5. When the amplitude of V_(RFDET) is less than about forty millivolts, the overall transfer function of RF detector 20 is I_(DET)=k*(W₁+W₂+W₃+W₄+W₅)*V_(RFDET) ², approximately, where W₁-W₅ are the respective widths of MOSFETs NF₁ through NF₅. When high frequency signal V_(RFDET) reaches an amplitude of about forty millivolts, detection current IS₁ reaches its maximum value of (I_(MAX1)−I0₁), and further increases in amplitude do not result in further increases in IS₁. Hence, in a range above about forty millivolts, gain stage GS₁ makes no further contribution to I_(DET). From forty to about one hundred ten millivolts, only gain stages GS₂-GS₅ contribute to the total transconductance of RF detector 20, so I_(DET)=k*(W₂+W₃+W₄+W₅)*V_(RFDET) ², approximately. In a similar fashion, as other V_(RFDET) breakpoints listed in Table I are reached, successive detection currents IS₂-IS₄ reach their maximum levels (I_(MAX2)−I0₂), . . . , (I_(MAX4)−I0₄) to reduce the effective overall transconductance of RF detector 20 in successive steps until V_(RFDET) reaches eight hundred ninety millivolts. At higher V_(RFDET) amplitudes, only gain stage GS₅ contributes to further increases in I_(DET). In effect, as the nonlinearity of each stage's transconductance increases, RF detector 20 compensates by successively reducing the number of stages that actively contribute to the overall transconductance. The result is a linear transfer function in which I_(DET) is proportional to V_(RFDET). It is evident that the number of gain stages and/or the breakpoint amplitudes may vary depending on the linearity desired in the overall transfer function, the degree of nonlinearity of each gain stage and other factors.

[0028] Hence, the present invention provides an integrated circuit operating as an RF detector that has a first gain stage whose input monitors a high frequency signal and routes a first detection current to a node. A second gain stage includes a first current source that supplies a bias current indicative of a predefined amplitude of the high frequency signal. An input of the second gain stage monitors the high frequency signal to route a portion of the bias current to the node as a second detection current. The second detection current is limited to the bias current when the high frequency signal is greater than the predefined amplitude to compensate for a nonlinearity in a transconductance of the second gain stage. Hence, above the predefined amplitude of the high frequency signal, the detector circuit's overall transconductance is modified to compensate for the nonlinearity. Other gain stages operate in a similar fashion to successively apply different transconductances to different amplitude ranges of the high frequency signal, allowing the detector circuit to compensate for nonlinearities in the transconductances of the other gain stages. The result is a substantially linear transfer function in which the overall detection current is proportional to the high frequency signal. Moreover, the detection function is provided on an integrated circuit using standard CMOS processing and without requiring external Schottky diodes or similar devices. 

What is claimed is:
 1. An integrated detector circuit, comprising: a first gain stage having an input that monitors a high frequency signal for routing a first detection current to a node; and a second gain stage including a first current source for supplying a bias current indicative of a predefined amplitude of the high frequency signal, and having an input for monitoring the high frequency signal to route a portion of the bias current to the node as a second detection current, wherein the second detection current is limited to the bias current when the high frequency signal is greater than the predefined amplitude.
 2. The integrated detector circuit of claim 1, wherein the second gain stage includes a first transistor having a control electrode coupled for receiving the high frequency signal and a first conduction electrode coupled to the node for supplying the second detection current.
 3. The integrated detector circuit of claim 2, wherein the current source includes a second transistor having a conduction electrode coupled to a second conduction electrode of the first transistor.
 4. The integrated detector circuit of claim 3, wherein the first transistor is an n-channel metal oxide semiconductor field effect transistor (MOSFET) having a gate that functions as the control electrode and the second transistor is a p-channel MOSFET having a drain that functions as the conduction electrode.
 5. The integrated detector circuit of claim 2, further comprising a third gain stage having an input that monitors the high frequency signal for routing a third detection current to the node.
 6. The integrated detector circuit of claim 5, wherein the first gain stage includes: a second current source for supplying a second bias current; and a second transistor having a control electrode coupled for receiving the high frequency signal and a first conduction electrode for routing a portion of the second bias current to the node as the second detection current.
 7. The integrated detector circuit of claim 6, wherein the first and second transistors are scaled to a ratio and the first and second detection currents are scaled to the ratio when the high frequency signal is zero.
 8. The integrated detector circuit of claim 1, further comprising an amplifier having a first input coupled to receive a reference signal, a second input coupled to the node, and an output for maintaining the node at a predetermined potential.
 9. The integrated detector circuit of claim 1, wherein the high frequency signal operates at a frequency greater than four hundred megahertz.
 10. A detector circuit, comprising gain stages that include current sources for establishing maximum current levels in the gain stages at corresponding amplitudes of a high frequency signal, wherein the gain stages function with transfer functions that convert the high frequency signal to detection currents for summing at a common node to produce an output detection signal as a substantially linear function of the high frequency signal, wherein the detection currents reach the maximum current levels at the corresponding amplitudes to compensate for nonlinearities in the transfer functions.
 11. The detector circuit of claim 10, wherein the gain stages include transistors whose control electrodes are coupled to an input of the detector circuit and whose sources are coupled to the common node for providing the detection currents.
 12. The detector circuit of claim 10, further comprising: an amplifier having a first input for receiving a reference voltage and a second input coupled to the common node; and a transistor having a control electrode coupled to an output of the amplifier and a conduction electrode coupled to the common node.
 13. A method of detecting a high frequency signal, comprising the steps of: amplifying a high frequency signal with a first transconductance to produce a first detection current; amplifying the high frequency signal with a second transconductance to produce a second detection current for summing with the first detection current to produce an output signal; and limiting the first detection current to a constant value to compensate for a nonlinearity in the second transconductance when the high frequency signal is greater than a predefined amplitude.
 14. The method of claim 13, wherein the step of limiting includes the steps of: generating a bias current with a current source; and routing a portion of the bias current through a transistor with the high frequency signal to produce the first detection current.
 15. The method of claim 13, further comprising the step of summing the first and second detection currents at a node to produce an output signal.
 16. The method of claim 15, further comprising the steps of: developing a reference voltage with a reference current; and amplifying a difference between the reference voltage and a potential at the node to produce a correction signal that maintains the potential at the node substantially constant.
 17. The method of claim 11, wherein the step of amplifying a high frequency signal with a first transconductance includes the step of converting a signal operating at a frequency greater than about four hundred megahertz to the first detection current.
 18. A detector circuit, comprising: a current source for providing a bias current; a first transistor operating in response to a high frequency signal and having a first width and a first conduction electrode coupled to a node for producing a portion of the bias current as a first detection current; and a second transistor operating in response to the high frequency signal, having a second width less than the first width, and having a first conduction electrode coupled to the node for producing a second detection current for summing with the first detection current to produce an output signal.
 19. The detector circuit of claim 18, wherein the first detection current has a value substantially equal to the bias current when the high frequency signal is greater than a predefined amplitude to compensate for a nonlinearity in a transfer function of the second transistor.
 20. The detector circuit of claim 19, further comprising a third transistor having a conduction electrode coupled to a second conduction electrode of the first transistor to supply the bias current. 